el-remmen
Moderator Emeritus
If a roll is warranted, I would base the DC on the overall situation, which includes the goal and approach. I don't base the DC on how "good" or "entertaining" the roleplay is. In this case, I would call for an opposed roll rather than a set DC, something like: "Roll a Charisma(Deception) check. You can roll with advantage since you are wearing a guard outfit. The guard will be rolling a Wisdom(Insight) check. If you succeed, the guard is none the wiser lets you pass. If you fail, the guard is going to think you are full of it and react accordingly."
I am much more likely to use a fail forward approach with this kind of social check, esp. when a roll misses by less than 5 (or something). So in the example above, if the check fails by not too much, I might have the guard be suspicious and call for his boss to get confirmation (giving the PCs a chance to come up with another or follow-up option while they wait) or I might have him ask a follow-up question, "Oh you guys are from the north barracks? How's Wally holding up? That owlbear wound he suffering getting any better?"
That said, while I get why you want to tell the PCs ahead of time the results of failure, I prefer to keep them hidden until they actually happen because 1. it keeps my options open, and 2. I think the possibilities of what happens with a failure are in the players hands to figure out when assessing risk (though I would of course fill in gaps of knowledge that the character would know even if the player doesn't - so if the PCs decide that the worst that would happen is being questioned, I might let them know that they had heard the Baron had put out an order for summary execution for any outsiders found on the property - I am not trying to "get" them).
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